1. Field of the Invention
The present invention generally relates to a circuit for an optical Wavelength-Division Multiplexing (WDM) system, and in particular to an optical circuit such as a wavelength Add/Drop multiplexer and an optical WDM network including the same.
2. Description of the Related Art
Wavelength-Division Multiplexing abbreviated as WDM is the current favorite multiplexing technology for optical communication networks. In such an optical WDM communications network, a wavelength Add/Drop circuit is needed to drop the optical signal on a predetermined wavelength and add a new data stream to the same wavelength at the location thereof.
A conventional Add/Drop circuit has been proposed by Miyakawa et al. in an article "Suppression of cross talk penalty in optical ADM by high speed polarization scrambler" (B-1087, a publication of the communications society of IEICE, 1996). The Add/Drop circuit inserted on a physical fiber link is composed of two optical circulators connected in cascade and two wavelength filters which are connected to the optical circulators, respectively. A first combination of the first circulator and the first filter is used to drop an optical signal on the corresponding wavelength of the first filter and a second combination is used to add a new optical signal to the same wavelength.
Another conventional Add/Drop circuit has been proposed by Miyaji et al. in an article "A study on wavelength variation in optical ADM network" (B-1086, a publication of the communications society of IEICE, 1996). The Add/Drop circuit is composed of a WDM coupler as a demultiplexer, one switch per wavelength, and another WDM coupler as a multiplexer. If one of the switches is opened, the signal on the corresponding wavelength is dropped locally and a new signal is added to the same wavelength at this Add/drop circuit.
However, in the case where a plurality of wavelengths are dropped and added, the former circuit needs two circulators and two filters for each wavelength, resulting in increased amount of hardware. In the latter circuit, since two WDM couplers are needed, it is difficult to make the size of the circuit small, especially in the case of a large number of wavelengths.